Multi-layered ceramic structures are, of course, well known. In these ceramic structures, a plurality of ceramic sheets in the green, unsintered state are stacked and laminated together. Each of the sheets usually has a metallic conducting pattern or circuitry on its surface. Interconnections between circuitry on different sheets are achieved by vias. Finally, the stacked and laminated ceramic structure is sintered to form a substrate. Such substrates are disclosed in, for example, Arrhenius U.S. Pat. No. 3,423,517 and Theobald et al. U.S. Pat. No. 3,723,176, which are incorporated by reference herein.
It has been found, however, that defects, such as debris and line spreading (or blooming) are generated during the handling and stacking operations. These defects usually manifest themselves as electrical shorts and opens. Since these defects may not be discovered until after the multi-layered ceramic (MLC) substrate is formed, thereby leading to a defective and nonrepairable substrate, it would be desirable to avoid them if possible. It is believed that these defects are due, at least in part, to the fact that the conducting metal pattern or circuitry is not flush with the surface of the ceramic sheet before each of the green sheets is stacked and laminated.
Various alternative methods of manufacturing an electrical substrate have been proposed. In Parfet U.S. Pat. No. 3,800,020, which is incorporated by reference herein, there is disclosed a process wherein powdered metal, on a heated sheet of thermoplastic resin, is forced into the sheet with a die having the embossed circuit pattern on the surface. The remaining powdered metal not forced into the sheet is removed.
Elderbaum U.S. Pat. No. 3,882,059, which is incorporated by reference herein, discloses a process for producing capacitors comprising the steps of placing a conductive metal pattern on a release surface, placing a ceramic sheet on the pattern and release surface and then peeling away the ceramic sheet and pattern from the release surface.
Yokouchi et al. U.S. Pat. No. 4,346,516, which is incorporated by reference herein, discloses forcing metal balls into a green ceramic sheet. The metal balls serve the function of vias in that they connect the conductive wiring patterns of upper and lower layers.
In Noack U.S. Pat. No. 3,247,573, which is incorporated by reference herein, the conductive elements are formed by casting a ceramic slurry around the elements.
R. Acosta et al. in "Planarization Of Copper Circuits For Multilevel Ceramic Package", TBM Technical Dislosure Bulletin, Vol. 26, No. 10B, 5378 (March 1984), which is incorporated by reference herein, discloses the embedding of copper conductors in a polymeric matrix by heating the polymer and then pressing the copper conductors into the polymeric matrix. Thereafter, an additional polymeric layer may be applied.
Desai et al. in "Screen Printing On Indented Ceramic Green Sheets", IBM Technical Disclosure Bulletin, Vol. 16, No. 11, 3561 (April 1974), discloses hot stamping an indentation into a ceramic green sheet which has been previously coated with polyvinyl alcohol. Thereafter, a paste is squeegeed onto the ceramic green sheet to fill the indentation. Desai et at. in "Adhesion Promotion To Green Ceramic Sheets", IBM Technical Disclosure Bulletin, Vol. 16, No. 11, 3563 (April 1974), discloses the coating of a ceramic sheet with an organic resin. Both of these disclosures are incorporated by reference herein.
Notwithstanding the work of those skilled in the art, as illustrated by the above references, there does not yet appear a solution to the problem of defects in MLC substrates.
Accordingly, it is an object of this invention to have a way to reduce the defects in MLC substrates.
It is a further object of the invention to have such a way of reducing defects in MLC substrates which is both effective and practical in use.
These and other objects of the invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawing.